<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    h1,h2 {
      color: yellow;
    }
    .text-center {
    text-align: center;
    }

    h1,span {
      background: blue;
    }
    /* 去掉 A标签下划线 */
    a {
      text-decoration: none;
    }
    .success {
      border: solid 1px green;
      color: green;
    }
    .error {
      border: solid 1px red;
      color: red;
    }

    .bg {
      background: red;
    }
    .mb20 {
      margin-bottom: 20px
    }
    .mb10 {
      margin-bottom: 10px
    }
    
    
  </style>
</head>
<body>
  <h1 class="text-center">houdunren.com</h1>
  <h1 class="text-center">houdunren.com</h1>
  <h1 class="text-center">houdunren.com</h1>
  <span>hdcss.com</span> 
  <h2>后盾人</h2>
  <span>hdcms.com</span>
  <a href="">后盾人</a>
  
  <div class="success mb20 bg">操作成功</div>
  <div class="success mb20 bg">操作成功</div>
  <div class="error">表单错误</div>
</body>
</html>